Twinstar, the Drosophila homolog of cofilin/ADF, is required for planar cell polarity patterning

Scale-type-specific requirement for the mosquito Aedes aegypti Spindle-F homologue by regulating microtubule organization

Insect epithelial cells contain unique cellular extensions such as bristles, hairs, and scales. In contrast to bristle and hair, which are not divergent in their shape, scale morphology shows high diversity. In our attempt to characterize the role of the insect-specific gene, Spindle-F (spn-F), in mosquito development, we revealed a scale-type specific requirement for the mosquito Aedes aegypti spn-F homologue. Using CRISPR-Cas9, we generated Ae-spn-F mutants and found that Ae-spn-F is an essential gene, but we were able to recover a few adult escapers. These escapers could not fly nor move, and died after 3 to 4 days. We found that in Ae-spn-F mutants, only the tip part of the bristle was affected with bulbous with misoriented ribs. We also show that in Ae-spn-F mutants, only in falcate scales, which are curved with a sharp or narrowly rounded apex, and not in other scale types, the tip region is strongly affected. Our analysis also revealed that in contrast to Drosophila spn-F, which show strong defects in both the actin and microtubule (MT) network in the bristle, the Ae-spn-F gene is required only for MT organization in scales and bristles. In summary, our results reveal that Ae-spn-F is required for shaping tapered epithelial cellular extension structures, namely, the bristle and falcate scales by affecting MT organization.

Cofilin Loss in Drosophila Muscles Contributes to Muscle Weakness through Defective Sarcomerogenesis during Muscle Growth

Sarcomeres, the fundamental contractile units of muscles, are conserved structures composed of actin thin filaments and myosin thick filaments. How sarcomeres are formed and maintained is not well understood. Here, we show that knockdown of Drosophila cofilin (DmCFL), an actin depolymerizing factor, disrupts both sarcomere structure and muscle function. The loss of DmCFL also results in the formation of sarcomeric protein aggregates and impairs sarcomere addition during growth. The activation of the proteasome delays muscle deterioration in our model. Furthermore, we investigate how a point mutation in CFL2 that causes nemaline myopathy (NM) in humans affects CFL function and leads to the muscle phenotypes observed in vivo. Our data provide significant insights to the role of CFLs during sarcomere formation, as well as mechanistic implications for disease progression in NM patients.

How sarcomeres are added and maintained in a growing muscle cell is unclear. Balakrishnan et al. observed that DmCFL loss in growing muscles affects sarcomere size and addition through unregulated actin polymerization. This results in a collapse of sarcomere and muscle structure, formation of large protein aggregates, and muscle weakness.

Coming to Consensus: A Unifying Model Emerges for Convergent Extension

Cell motility is a widespread biological property that is best understood in the context of individual cell migration. Indeed, studies of migration in culture have provided tremendous insight into the signals and mechanics involved and have laid the foundation for our understanding of similar migrations by larger cellular collectives. By contrast, our understanding of another flavor of movement, cell intercalation during convergent extension, is only now emerging. Here, we integrate divergent findings related to intercalation in different settings into a unifying model, paying attention to how this model does and does not resemble current models for directed cell migration.

Structure, dynamics, and biochemical characterization of ADF/cofilin Twinstar from Drosophilamelanogaster

BACKGROUND

Twinstar is an ADF/cofilin family protein, which is expressed by the tsr gene in Drosophila melanogaster. Twinstar is one of the main regulators of actin cytoskeleton remodelling and is essential for vital cellular processes like cytokinesis and endocytosis.

METHODS

We have characterized the structure and dynamics of Twinstar by solution NMR spectroscopy, the interaction of Twinstar with rabbit muscle actin by ITC, and biochemical activities of Twinstar through different biochemical assays using fluorescence spectroscopy and ultra-centrifugation.

RESULTS

The solution structure of Twinstar shows characteristic ADF-H fold with well-formed G/F-site and F-site for interaction with actin. The structure possesses an extended F-loop, which is rigid at the base, but flexible towards its apical region. Twinstar shares similar dynamics for the G/F-site with C. elegans homologs, UNC-60A and UNC-60B. However, the dynamics of its F-loop are different from its C. elegans homologs. Twinstar shows strong affinity for ADP-G-Actin and ATP-G-Actin with K_ds of ~7.6 nM and ~0.4 μM, respectively. It shows mild F-actin depolymerizing activity and stable interaction with F-actin with a K_d of ~5.0 μM. It inhibits the rate of the nucleotide exchange in a dose dependent manner.

CONCLUSION

On the basis of structure, dynamics, and biochemical activity, Twinstar can be taken to execute its biochemical role by facilitating directional growth and maintenance of length of actin filaments.

GENERAL SIGNIFICANCE

This study characterizes the structure, backbone dynamics, and biochemical activities of Twinstar of Drosophila, which provides an insight into the regulation of actin dynamics in the member of phylum insecta.

Tissue morphodynamics: Translating planar polarity cues into polarized cell behaviors

The ability of cells to collectively orient and align their behaviors is essential in multicellular organisms for unidirectional cilia beating, collective cell movements, oriented cell divisions, and asymmetric cell fate specification. The planar cell polarity pathway coordinates a vast and diverse array of collective cell behaviors by intersecting with downstream pathways that regulate cytoskeletal dynamics and intercellular signaling. How the planar polarity pathway translates directional cues to produce polarized cell behaviors is the focus of this review.

Planar polarization of Vangl2 in the vertebrate neural plate is controlled by Wnt and Myosin II signaling

The vertebrate neural tube forms as a result of complex morphogenetic movements, which require the functions of several core planar cell polarity (PCP) proteins, including Vangl2 and Prickle. Despite the importance of these proteins for neurulation, their subcellular localization and the mode of action have remained largely unknown. Here we describe the anteroposterior planar cell polarity (AP-PCP) of the cells in the Xenopus neural plate. At the neural midline, the Vangl2 protein is enriched at anterior cell edges and that this localization is directed by Prickle, a Vangl2-interacting protein. Our further analysis is consistent with the model, in which Vangl2 AP-PCP is established in the neural plate as a consequence of Wnt-dependent phosphorylation. Additionally, we uncover feedback regulation of Vangl2 polarity by Myosin II, reiterating a role for mechanical forces in PCP. These observations indicate that both Wnt signaling and Myosin II activity regulate cell polarity and cell behaviors during vertebrate neurulation.

Morphogenesis of the mouse neural plate depends on distinct roles of cofilin 1 in apical and basal epithelial domains

The genetic control of mammalian epithelial polarity and dynamics can be studied in vivo at cellular resolution during morphogenesis of the mouse neural tube. The mouse neural plate is a simple epithelium that is transformed into a columnar pseudostratified tube over the course of ∼ 24 h. Apical F-actin is known to be important for neural tube closure, but the precise roles of actin dynamics in the neural epithelium are not known. To determine how the organization of the neural epithelium and neural tube closure are affected when actin dynamics are blocked, we examined the cellular basis of the neural tube closure defect in mouse mutants that lack the actin-severing protein cofilin 1 (CFL1). Although apical localization of the adherens junctions, the Par complex, the Crumbs complex and SHROOM3 is normal in the mutants, CFL1 has at least two distinct functions in the apical and basal domains of the neural plate. Apically, in the absence of CFL1 myosin light chain does not become phosphorylated, indicating that CFL1 is required for the activation of apical actomyosin required for neural tube closure. On the basal side of the neural plate, loss of CFL1 has the opposite effect on myosin: excess F-actin and myosin accumulate and the ectopic myosin light chain is phosphorylated. The basal accumulation of F-actin is associated with the assembly of ectopic basal tight junctions and focal disruptions of the basement membrane, which eventually lead to a breakdown of epithelial organization.

The diaphanous gene of Drosophila interacts antagonistically with multiple wing hairs and plays a key role in wing hair morphogenesis

The Drosophila wing is covered by an array of distally pointing hairs that has served as a key model system for studying planar cell polarity (PCP). The adult cuticular hairs are formed in the pupae from cell extensions that contain extensive actin filaments and microtubules. The importance of the actin cytoskeleton for hair growth and morphogenesis is clear from the wide range of phenotypes seen in mutations in well-known actin regulators. Formin proteins promote the formation of long actin filaments of the sort thought to be important for hair growth. We report here that the formin encoding diaphanous (dia) gene plays a key role in hair morphogenesis. Both loss of function mutations and the expression of a constitutively active Dia led to cells forming both morphologically abnormal hairs and multiple hairs. The conserved frizzled (fz)/starry night (stan) PCP pathway functions to restrict hair initiation and activation of the cytoskeleton to the distal most part of wing cells. It also ensures the formation of a single hair per cell. Our data suggest that the localized inhibition of Dia activity may be part of this mechanism. We found the expression of constitutively active Dia greatly expands the region for activation of the cytoskeleton and that dia functions antagonistically with multiple wing hairs (mwh), the most downstream member of the fz/stan pathway. Further we established that purified fragments of Dia and Mwh could be co-immunoprecipitated suggesting the genetic interaction could reflect a direct physical interaction.

Arabidopsis AIP1-2 restricted by WER-mediated patterning modulates planar polarity

The coordination of cell polarity within the plane of the tissue layer (planar polarity) is crucial for the development of diverse multicellular organisms. Small Rac/Rho-family GTPases and the actin cytoskeleton contribute to planar polarity formation at sites of polarity establishment in animals and plants. Yet, upstream pathways coordinating planar polarity differ strikingly between kingdoms. In the root of Arabidopsis thaliana, a concentration gradient of the phytohormone auxin coordinates polar recruitment of Rho-of-plant (ROP) to sites of polar epidermal hair initiation. However, little is known about cytoskeletal components and interactions that contribute to this planar polarity or about their relation to the patterning machinery. Here, we show that ACTIN7 (ACT7) represents a main actin isoform required for planar polarity of root hair positioning, interacting with the negative modulator ACTIN-INTERACTING PROTEIN1-2 (AIP1-2). ACT7, AIP1-2 and their genetic interaction are required for coordinated planar polarity of ROP downstream of ethylene signalling. Strikingly, AIP1-2 displays hair cell file-enriched expression, restricted by WEREWOLF (WER)-dependent patterning and modified by ethylene and auxin action. Hence, our findings reveal AIP1-2, expressed under control of the WER-dependent patterning machinery and the ethylene signalling pathway, as a modulator of actin-mediated planar polarity.

Summary: Planar polarity in Arabidopsis is shaped by ACTIN-INTERACTING PROTEIN1-2, which is under the control of WEREWOLF-dependent patterning and ethylene signalling.

Changes in ADF/destrin expression in the development of hair cells following Atoh1-induced ectopic regeneration

The aim of this study was to investigate the effects of actin depolymerizing factor (ADF)/destrin and position changes of kinetosomes in the development of hair cells following Atoh1-induced ectopic regeneration in the basilar membrane of mice. We observed through immunofluorescence at various time-points the expression of ADF/destrin and the specific kinetosome marker, γ-tubulin, in hair cells following ectopic regeneration induced by adenovirus transfection, overexpression of Atoh1 and in vitro culture. Changes of ADF/destrin distribution and kinetosome position during in vitro culture of new hair cells [Myo7a(+)] following Atoh1-induced ectopic regeneration are consistent with the changes in ADF/destrin expression and the polar migration of kinetosomes in hair cells of the cochlear sensory epithelium in normal development. ADF/destrin is involved in the development of the auditory epithelium and the development and structural rearrangement of ectopically regenerated hair cells in mammals. The kinetosomes of hair cells following Atoh1-induced ectopic regeneration show positional changes in vitro at different time-points.

Proteomic biomarkers for ageing the mosquito Aedes aegypti to determine risk of pathogen transmission

Biomarkers of the age of mosquitoes are required to determine the risk of transmission of various pathogens as each pathogen undergoes a period of extrinsic incubation in the mosquito host. Using the 2-D Difference Gel Electrophoresis (2-D DIGE) procedure, we investigated the abundance of up to 898 proteins from the Yellow Fever and dengue virus vector, Aedes aegypti, during ageing. By applying a mixed-effects model of protein expression, we identified five common patterns of abundance change during ageing and demonstrated an age-related decrease in variance for four of these. This supported a search for specific proteins with abundance changes that remain tightly associated with ageing for use as ageing biomarkers. Using MALDI-TOF/TOF mass spectrometry we identified ten candidate proteins that satisfied strict biomarker discovery criteria (identified in two out of three multivariate analysis procedures and in two cohorts of mosquitoes). We validated the abundances of the four most suitable candidates (Actin depolymerising factor; ADF, Eukaryotic initiation factor 5A; eIF5A, insect cuticle protein Q17LN8, and Anterior fat body protein; AFP) using semi-quantitative Western analysis of individual mosquitoes of six ages. The redox-response protein Manganese superoxide dismutase (SOD2) and electron shuttling protein Electron transfer oxidoreductase (ETO) were subject to post-translational modifications affecting their charge states with potential effects on function. For the four candidates we show remarkably consistent decreases in abundance during ageing, validating initial selections. In particular, the abundance of AFP is an ideal biomarker candidate for whether a female mosquito has lived long enough to be capable of dengue virus transmission. We have demonstrated proteins to be a suitable class of ageing biomarkers in mosquitoes and have identified candidates for epidemiological studies of dengue and the evaluation of new disease reduction projects targeting mosquito longevity.

Cofilin and Vangl2 cooperate in the initiation of planar cell polarity in the mouse embryo

The planar cell polarity (PCP; non-canonical Wnt) pathway is required to orient the cells within the plane of an epithelium. Here, we show that cofilin 1 (Cfl1), an actin-severing protein, and Vangl2, a core PCP protein, cooperate to control PCP in the early mouse embryo. Two aspects of planar polarity can be analyzed quantitatively at cellular resolution in the mouse embryo: convergent extension of the axial midline; and posterior positioning of cilia on cells of the node. Analysis of the spatial distribution of brachyury(+) midline cells shows that the Cfl1 mutant midline is normal, whereas Vangl2 mutants have a slightly wider midline. By contrast, midline convergent extension fails completely in Vangl2 Cfl1 double mutants. Planar polarity is required for the posterior positioning of cilia on cells in the mouse node, which is essential for the initiation of left-right asymmetry. Node cilia are correctly positioned in Cfl1 and Vangl2 single mutants, but cilia remain in the center of the cell in Vangl2 Cfl1 double mutants, leading to randomization of left-right asymmetry. In both the midline and node, the defect in planar polarity in the double mutants arises because PCP protein complexes fail to traffic to the apical cell membrane, although other aspects of apical-basal polarity are unaffected. Genetic and pharmacological experiments demonstrate that F-actin remodeling is essential for the initiation, but not maintenance, of PCP. We propose that Vangl2 and cofilin cooperate to target Rab11(+) vesicles containing PCP proteins to the apical membrane during the initiation of planar cell polarity.

AIP1 acts with cofilin to control actin dynamics during epithelial morphogenesis

During epithelial morphogenesis, cells not only maintain tight adhesion for epithelial integrity but also allow dynamic intercellular movement to take place within cell sheets. How these seemingly opposing processes are coordinated is not well understood. Here, we report that the actin disassembly factors AIP1 and cofilin are required for remodeling of adherens junctions (AJs) during ommatidial precluster formation in Drosophila eye epithelium, a highly stereotyped cell rearrangement process which we describe in detail in our live imaging study. AIP1 is enriched together with F-actin in the apical region of preclusters, whereas cofilin displays a diffuse and uniform localization pattern. Cofilin overexpression completely rescues AJ remodeling defects caused by AIP1 loss of function, and cofilin physically interacts with AIP1. Pharmacological reduction of actin turnover results in similar AJ remodeling defects and decreased turnover of E-cadherin, which also results from AIP1 deficiency, whereas an F-actin-destabilizing drug affects AJ maintenance and epithelial integrity. Together with other data on actin polymerization, our results suggest that AIP1 enhances cofilin-mediated actin disassembly in the apical region of precluster cells to promote remodeling of AJs and thus intercellular movement, but also that robust actin polymerization promotes AJ general adhesion and integrity during the remodeling process.

Cofilin/Twinstar phosphorylation levels increase in response to impaired coenzyme a metabolism

Coenzyme A (CoA) is a pantothenic acid-derived metabolite essential for many fundamental cellular processes including energy, lipid and amino acid metabolism. Pantothenate kinase (PANK), which catalyses the first step in the conversion of pantothenic acid to CoA, has been associated with a rare neurodegenerative disorder PKAN. However, the consequences of impaired PANK activity are poorly understood. Here we use Drosophila and human neuronal cell cultures to show how PANK deficiency leads to abnormalities in F-actin organization. Cells with reduced PANK activity are characterized by abnormally high levels of phosphorylated cofilin, a conserved actin filament severing protein. The increased levels of phospho-cofilin coincide with morphological changes of PANK-deficient Drosophila S2 cells and human neuronal SHSY-5Y cells. The latter exhibit also markedly reduced ability to form neurites in culture--a process that is strongly dependent on actin remodeling. Our results reveal a novel and conserved link between a metabolic biosynthesis pathway, and regulation of cellular actin dynamics.

The frizzled/stan pathway and planar cell polarity in the Drosophila wing

Drosophila has been the key model system for studies on planar cell polarity (PCP). The rich morphology of the insect exoskeleton contains many structures that display PCP. Among these are the trichomes (cuticular hairs) that cover much of the exoskeleton, sensory bristles, and ommatidia. Many genes have been identified that must function for the development of normal PCP. Among these are the genes that comprise the frizzled/starry night (fz/stan) and dachsous/fat pathways. The mechanisms that underlie the function of the fz/stan pathway are best understood. All of the protein products of these genes accumulate asymmetrically in wing cells and there is good evidence that this involves local intercellular signaling between protein complexes on the distal edge of one cell and the juxtaposed proximal edge of its neighbor. It is thought that a feedback system, directed transport, and stabilizing protein-protein interactions mediate the formation of distal and proximal protein complexes. These complexes appear to recruit downstream proteins that function to spatially restrict the activation of the cytoskeleton in wing cells. This leads to the formation of the array of distally pointing hairs found on wings.

Rho GTPase controls Drosophila salivary gland lumen size through regulation of the actin cytoskeleton and Moesin

Generation and maintenance of proper lumen size is important for tubular organ function. We report on a novel role for the Drosophila Rho1 GTPase in control of salivary gland lumen size through regulation of cell rearrangement, apical domain elongation and cell shape change. We show that Rho1 controls cell rearrangement and apical domain elongation by promoting actin polymerization and regulating F-actin distribution at the apical and basolateral membranes through Rho kinase. Loss of Rho1 resulted in reduction of F-actin at the basolateral membrane and enrichment of apical F-actin, the latter accompanied by enrichment of apical phosphorylated Moesin. Reducing cofilin levels in Rho1 mutant salivary gland cells restored proper distribution of F-actin and phosphorylated Moesin and rescued the cell rearrangement and apical domain elongation defects of Rho1 mutant glands. In support of a role for Rho1-dependent actin polymerization in regulation of gland lumen size, loss of profilin phenocopied the Rho1 lumen size defects to a large extent. We also show that Ribbon, a BTB domain-containing transcription factor functions with Rho1 in limiting apical phosphorylated Moesin for apical domain elongation. Our studies reveal a novel mechanism for controlling salivary gland lumen size, namely through Rho1-dependent actin polymerization and distribution and downregulation of apical phosphorylated Moesin.

Epidermal wound repair is regulated by the planar cell polarity signaling pathway

The mammalian PCP pathway regulates diverse developmental processes requiring coordinated cellular movement, including neural tube closure and cochlear stereociliary orientation. Here, we show that epidermal wound repair is regulated by PCP signaling. Mice carrying mutant alleles of PCP genes Vangl2, Celsr1, PTK7, and Scrb1, and the transcription factor Grhl3, interact genetically, exhibiting failed wound healing, neural tube defects, and disordered cochlear polarity. Using phylogenetic analysis, ChIP, and gene expression in Grhl3(-)(/-) mice, we identified RhoGEF19, a homolog of a RhoA activator involved in PCP signaling in Xenopus, as a direct target of GRHL3. Knockdown of Grhl3 or RhoGEF19 in keratinocytes induced defects in actin polymerization, cellular polarity, and wound healing, and re-expression of RhoGEF19 rescued these defects in Grhl3-kd cells. These results define a role for Grhl3 in PCP signaling and broadly implicate this pathway in epidermal repair.

Developmental expression of the actin depolymerizing factor ADF in the mouse inner ear and spiral ganglia

Hair cells, the inner ear's sensory cells, are characterized by tens to hundreds of actin-rich stereocilia that form the hair bundle apparatus necessary for mechanoelectrical transduction. Both the number and length of actin filaments are precisely regulated in stereocilia. Proper cochlear and vestibular function also depends on actin filaments in nonsensory supporting cells. The formation of actin filaments is a dynamic, treadmill-like process in which actin-binding proteins play crucial roles. However, little is known about the presence and function of actin binding molecules in the inner ear, which set up, and maintain, actin-rich structures and regulate actin turnover. Here we examined the expression and subcellular location of the actin filament depolymerizing factor (ADF) in the cochlea and vestibular organs. By means of immunocytochemistry and confocal microscopy, we analyzed whole-mount preparations and cross-sections in fetal and postnatal mice (E15-P26). We found a transient ADF expression in immature hair cells of the organ of Corti, the utricle, and the saccule. Interestingly, the stereocilia were not labeled. By P26, ADF expression was restricted to supporting cells. In addition, we localized ADF in presynaptic terminals of medio-olivocochlear projections after hearing onset. A small population of spiral ganglion neurons strongly expressed ADF. Based on their relative number, peripheral location within the ganglion, smaller soma size, and coexpression of neurofilament 200, we identified these cells as Type II spiral ganglion neurons. The developmentally regulated ADF expression suggests a temporally restricted function in the stereocilia and, thus, a hitherto undescribed role of ADF.

The small GTPase Rac1 regulates auditory hair cell morphogenesis

Morphogenesis of sensory hair cells, in particular their mechanotransduction organelle, the stereociliary bundle, requires highly organized remodeling of the actin cytoskeleton. The roles of Rho family small GTPases during this process remain unknown. Here we show that deletion of Rac1 in the otic epithelium resulted in severe defects in cochlear epithelial morphogenesis. The mutant cochlea was severely shortened with a reduced number of auditory hair cells and cellular organization of the auditory sensory epithelium was abnormal. Rac1 mutant hair cells also displayed defects in planar cell polarity and morphogenesis of the stereociliary bundle, including bundle fragmentation or deformation, and mispositioning or absence of the kinocilium. We further demonstrate that a Rac-PAK (p21-activated kinase) signaling pathway mediates kinocilium-stereocilia interactions and is required for cohesion of the stereociliary bundle. Together, these results reveal a critical function of Rac1 in morphogenesis of the auditory sensory epithelium and stereociliary bundle.

Rho1 has multiple functions in Drosophila wing planar polarity

The frizzled (fz) signaling/signal transduction pathway controls planar cell polarity in both vertebrates and invertebrates. Previous data implicated Rho1 as a component of the fz pathway in Drosophila but it was unclear how it functioned. The existence of a G Protein Binding-Formin Homology 3 (GBD-FH3) domain in Multiple Wing Hairs, a downstream component of the pathway suggested that Rho1 might function by binding to and activating Mwh. We re-examined the role of Rho1 in wing planar polarity and found that it had multiple functions. Aberrant Rho1 activity led to changes in the number of hairs formed, changes in cell shape and F-actin and changes in cellular junctions. Experiments that utilized Rho effector loop mutations argued that these phenotypes were mediated by effects of Rho1 on the cytoskeleton and not by effects on transcription. We found strong positive genetic interactions between Rho1 and mwh, that Rho1 regulated the accumulation of Mwh protein and that these two proteins could be co-immunoprecipitated. The Mwh GBD:FH3 domain was sufficient for co-immunoprecipitation with Rho1, consistent with this domain mediating the interaction. However, further experiments showed that Rho1 function in wing differentiation was not limited to interacting with Mwh. We established by genetic experiments that Rho1 could influence hair morphogenesis in the absence of mwh and that the disruption of Rho1 activity could interfere with the zig zag accumulation pattern of upstream fz pathway proteins. Thus, our results argue that in addition to its interaction with Mwh Rho1 has functions in wing planar polarity that are parallel to and upstream of fz. The upstream function may be an indirect one and associated with the requirement for normal apical basal polarity and adherens junctions for the accumulation of PCP protein complexes.

Rapid elevation of Inos and decreases in abundance of other proteins at pupal diapause termination in the flesh fly Sarcophaga crassipalpis

We analyzed changes in brain proteins 24 h after pupal diapause termination in Sarcophaga crassipalpis by a combination of 2-dimensional gel electrophoresis and mass spectrometry. The proteome analysis revealed significant changes in 20 proteins, 11 of which represented >or=2.5-fold changes. Three proteins were present only in the brains of diapausing pupae. Among the most abundant proteins that showed a change, 1 was more abundant, 7 were less abundant, and 2 were absent following diapause termination. The protein that increased in abundance following diapause termination showed highest identity to myo-inositol-1-phosphate synthase (Inos). Proteins that decreased at diapause termination included those showing highest identities to fatty acid binding protein, CG2331-PA, twinstar, catalase, and a histone. Proteins absent at diapause termination included ribosomal protein L17A and one unnamed protein. An increase of Inos protein level was confirmed using Western blot analysis. Attempts to terminate diapause by injection of several Inos-related metabolites failed, thus suggesting that the elevation of Inos at diapause termination is downstream of the physiological regulation that initiates development.

The flare gene, which encodes the AIP1 protein of Drosophila, functions to regulate F-actin disassembly in pupal epidermal cells

Adult Drosophila are decorated with several types of polarized cuticular structures, such as hairs and bristles. The morphogenesis of these takes place in pupal cells and is mediated by the actin and microtubule cytoskeletons. Mutations in flare (flr) result in grossly abnormal epidermal hairs. We report here that flr encodes the Drosophila actin interacting protein 1 (AIP1). In other systems this protein has been found to promote cofilin-mediated F-actin disassembly. In Drosophila cofilin is encoded by twinstar (tsr). We show that flr mutations result in increased levels of F-actin accumulation and increased F-actin stability in vivo. Further, flr is essential for cell proliferation and viability and for the function of the frizzled planar cell polarity system. All of these phenotypes are similar to those seen for tsr mutations. This differs from the situation in yeast where cofilin is essential while aip1 mutations result in only subtle defects in the actin cytoskeleton. Surprisingly, we found that mutations in flr and tsr also result in greatly increased tubulin staining, suggesting a tight linkage between the actin and microtubule cytoskeleton in these cells.

The cofilin pathway in breast cancer invasion and metastasis

Recent evidence indicates that metastatic capacity is an inherent feature of breast tumours and not a rare, late acquired event. This has led to new models of metastasis. The interpretation of expression-profiling data in the context of these new models has identified the cofilin pathway as a major determinant of metastasis. Recent studies indicate that the overall activity of the cofilin pathway, and not that of any single gene within the pathway, determines the invasive and metastatic phenotype of tumour cells. These results predict that inhibitors directed at the output of the cofilin pathway will have therapeutic benefit in combating metastasis.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

The Cdi/TESK1 kinase is required for Sevenless signaling and epithelial organization in the Drosophila eye

How cellular behaviors such as cell-to-cell communication, epithelial organization and cell shape reorganization are coordinated during development is poorly understood. The developing Drosophila eye offers an ideal model system to study these processes. Localized actin polymerization is required to constrict the apical surface of epithelial cells of the eye imaginal disc to maintain the refined arrangement of the developing ommatidia. The identity of each photoreceptor cell within the epithelium is determined by cell-to-cell contacts involving signal transduction events. The R7 photoreceptor cell requires the activity of the Sevenless RTK to adopt a proper cell fate. We performed an EP screen for negative regulators of this inductive process, and we identified the serine/threonine kinase Center divider (cdi) as a suppressor of the phenotype caused by an activated Sevenless receptor. Cdi is homologous to the human testis-specific kinase 1 (TESK1), a member of the LIM kinases involved in cytoskeleton control through ADF/cofilin phosphorylation. We have analyzed the effects of gain- and loss-of-function of cdi and found alterations in actin organization and in the adherens junctions proteins DE-cadherin and beta-catenin, as well as in Sevenless apical localization. Interference with the function of the ADF/cofilin phosphatase Slingshot (ssh), which antagonizes Cdi, also results in a suppression of signaling triggered by the Sevenless RTK. Our results reveal a critical interplay between the localization of molecules involved in epithelial organization and signal transduction.